Graphite heat spreaders have previously been proposed to move heat from discrete heat sources. One surface of the spreader is placed against the discrete source and the heat moves from it into the spreader. Heat is then conducted through the spreader and dissipated, from both surfaces of the spreader, to cooler adjacent surfaces by conduction or radiation, or to air by convection. Thick graphite spreaders, with high in-plane thermal conductivity, have a large cross-sectional area to conduct heat and can move more heat than a thin spreader made from the same material. However, graphite materials with a high in-plane thermal conductivity have relatively lower through-thickness thermal conductivity. This low through-thickness thermal conductivity impedes the flow through the thickness of the graphite and does not allow for maximum heat transfer through the spreader.
This problem can be overcome by embedding a thermal via in the graphite spreader at the location of the heat source. The thermal via is made from an isotropic material having a thermal conductivity higher than the through-thickness thermal conductivity of graphite. Candidate via materials include gold, silver, copper, aluminum, etc. and their various alloys. The thermal via is typically round and is sized so that its diameter is large enough to cover approximately the entire surface of the heat source. The end of the via contacts the heat source and heat flows into the via and through it. Heat is transferred into the graphite through the outside diameter of the via. The via transfers heat efficiently through the thickness of the graphite and makes the entire thickness of the graphite spreader available for maximum heat transfer. An example of previous teachings with regard to the use of vias in graphite heat spreaders is shown in U.S. Pat. No. 6,758,263 to Krassowski et al. and assigned to the assignee of the present invention, the details of which are incorporated herein by reference.
A thick graphite heat spreader, made from high in-plane thermal conductivity graphite and incorporating a thermal via is a more efficient spreader of heat than an equivalent all graphite, all copper or all aluminum heat spreader, and it is generally lighter than either an all copper or all aluminum heat spreader.
One particular application in which heat spreaders are used is in conjunction with printed circuit boards. Printed circuit boards are conventionally manufactured from dielectric materials such as glass fiber laminates (known as FR4 boards), polytetrafluoroethylene, and like materials. On one of the surfaces of such boards, or between layers of dielectric materials, are circuits, usually formed of copper. The circuits are commonly formed by photolithographic methods, sputtering, screen printing or the like (for circuits disposed between layers, the circuit is applied to the dielectric material before formation of the laminate). In addition, components such as LEDs, processors, or the like can be disposed on the surface of the boards, in contact with the circuits on the surface. Such components can generate substantial amounts of heat that must be dissipated for them to operate reliably and to their intended performance levels.
As a result of these heat generating components, the amount of heat that printed circuit boards must help dissipate can be significant. So called “thermal boards” are being developed where a layer of a heat spreading material such as copper or aluminum and alloys thereof is laminated with the dielectric material, on the surface opposite or in layers opposing that of the circuit and heat-generating components, to act as a heat spreader for the heat generated from the electronic components. It is important that the heat spreader be located such that at least one layer of dielectric material separates the heat spreader from the circuit(s), since the heat spreader materials are typically electrically conductive, and would interfere with the operation of the circuits if they were in contact.
There are several commercially available “thermal boards,” sometimes called metal core printed circuit boards (MCPCB), such as Insulated Metal Substrate™ thermal boards from The Bergquist Company, T-Clad™ thermal boards from Thermagon, HITT Plate boards from Denka, and Anotherm™ boards from TT Electronics. These thermal boards utilize thermally conductive dielectric layers, either through filling the dielectric layer with thermally conductive particles as in the case of the first three, or as in the case of the Anotherm solution, through a thin anodization layer on top of the aluminum heat spreader layer. The use of thermally conductive particles can be expensive, however, and the subsequent layer must be thick enough to ensure it is pin-hole free, adding to thermal resistance in the design. Additional limitations of this approach arise from the lack of flexibility to fabricate bent or non-planar circuit structures, and the fact that the dielectric material covers the entire surface of the heat spreader layer. The use of anodization as the dielectric layer attempts to overcome some of these issues, but forces the use of aluminum as its heat spreader layer, since copper cannot be anodized. Since the thermal conductivity of aluminum is significantly less than that of copper, this can be a thermal disadvantage. All of the foregoing approaches, however, can suffer soldering difficulties, since the same heat dissipation properties that are useful during the operation of the printed circuit board and components, inhibit an assembly process that requires point sources of heat for soldering (such as hot bar bonding, for example).
To overcome some, but not all of these issues, traditional printed circuit boards can be married to a separate metal heat spreader layer in a separate process. In this arrangement, the printed circuit board can be designed with thermal vias (typically drilled holes that are plated with copper) to conduct heat better through the unfilled dielectric layer of the printed circuit board, but these may only be used in applications where electrical isolation from component to component is not required.
Moreover, traditional heat spreading materials like copper or aluminum also add significant weight to the board, which is undesirable, and the coefficient of thermal expansion (CTE) of these materials may not closely match that of the glass fiber laminate, leading to physical stress on the printed circuit board with the application of heat and, potentially, delamination or cracking.
Additionally, since the heat spreader layer on these boards is comprised of an isotropic, thin (relative to its length and width) metal material, heat tends to flow through the thickness of the heat spreader readily, and resulting hot-spots can occur in the location directly opposite the heat source.
Another type of circuit assembly, referred to in the industry as a “flex circuit,” provides similar heat management problems. Flex circuits are formed by providing a circuit, such as a copper circuit as described above, on the surface of a polymer material, such as a polyimide or polyester, which functions as the dielectric layer. As the name suggests, these circuit materials are flexible and can even be provided as rolls of circuit materials that can later be married to a heat spreader layer like copper or aluminum. While very thin, the dielectric layer in flex circuits still adds appreciably to the thermal resistance in a given design, and suffers from some of the same issues observed in printed circuit boards. The use of thermal vias is still limited to electrically isolating applications as described previously. And as is apparent, the use of rigid metallic layers, such as of copper or aluminum, does not allow one to take advantage of the flexibility of flex circuits, where such a characteristic is important in an end-use application.
The use of a heat spreader formed of sheet(s) of compressed particles of exfoliated graphite can remedy many of the disadvantages encountered with the use of copper or aluminum heat spreaders, since such graphite materials provide the advantage of an 80% weight reduction compared to copper, while being able to match or even exceed the thermal conductivity of copper in the in-plane direction needed for heat spreading across the surface of a printed circuit board. In addition, graphite has an in-plane CTE of essentially zero, and lower stiffness than copper or aluminum, thus reducing thermal stresses at the graphite-dielectric bond.
While sheets of compressed particles of exfoliated graphite can even have the flexibility for use with flex circuits, the addition of a graphite-based heat spreader layer cannot counter all of the disadvantages arising from the location of a heat spreader such that one or more layers of dielectric material separate the heat spreader from the heat-generating components, with the reduced heat transfer from the components to the heat spreader layer.
Laminates in which one or more of the layers consist of flexible graphite sheets are known in the art. These structures find utility, for example, in gasket manufacture. See U.S. Pat. No. 4,961,991 to Howard. Howard discloses various laminate structures which contain metal or plastic sheets, bonded between sheets of flexible graphite. Howard discloses that such structures can be prepared by cold-working a flexible graphite sheet on both sides of a metal net and then press-adhering the graphite to the metal net. Howard also discloses placing a polymer resin coated cloth between two sheets of flexible graphite while heating to a temperature sufficient to soften the polymer resin, thereby bonding the polymer resin coated cloth between the two sheets of flexible graphite to produce a flexible graphite laminate. Similarly, Hirschvogel, U.S. Pat. No. 5,509,993, discloses flexible graphite/metal laminates prepared by a process which involves as a first step applying a surface active agent to one of the surfaces to be bonded. Mercuri, U.S. Pat. No. 5,192,605, also forms laminates from flexible graphite sheets bonded to a core material which may be metal, fiberglass or carbon. Mercuri deposits and then cures a coating of an epoxy resin and particles of a thermoplastic agent on the core material before feeding core material and flexible graphite through calender rolls to form the laminate.
In addition to their utility in gasket materials, graphite laminates also find utility as heat transfer or cooling apparatus. The use of various solid structures as heat transporters is known in the art. For example, Banks, U.S. Pat. Nos. 5,316,080 and 5,224,030 discloses the utility of diamonds and gas-derived graphite fibers, joined with a suitable binder, as heat transfer devices. Such devices are employed to passively conduct heat from a source, such as a semiconductor, to a heat sink.
In U.S. Pat. No. 6,758,263, Krassowski and Chen disclose the incorporation of a high conducting insert into a heat dissipating component such as a graphite heat sink base in order to conduct heat from a heat source through the thickness of the component, and from there in a planar direction. However, nothing in the Krassowski and Chen disclosure describes conducting heat from a heat source through layers of a relatively non-conductive material like a dielectric layer of a circuit assembly.
As noted, the graphite material preferred for use as the heat spreader material of this invention is sheets of compressed particles of exfoliated graphite, typically referred to as flexible graphite sheet material.
The following is a brief description of graphite and the manner in which it is typically processed to form flexible graphite sheet materials. Graphite, on a microscopic scale, is made up of layer planes of hexagonal arrays or networks of carbon atoms. These layer planes of hexagonally arranged carbon atoms are substantially flat and are oriented or ordered so as to be substantially parallel and equidistant to one another. The substantially-flat, parallel, equidistant sheets or layers of carbon atoms, usually referred to as graphene layers or basal planes, are linked or bonded together and groups thereof are arranged in crystallites. Highly-ordered graphite materials consist of crystallites of considerable size, the crystallites being highly aligned or oriented with respect to each other and having well ordered carbon layers. In other words, highly ordered graphites have a high degree of preferred crystallite orientation. It should be noted that graphites, by definition, possess anisotropic structures and thus exhibit or possess many characteristics that are highly directional, e.g., thermal and electrical conductivity and fluid diffusion.
Briefly, graphites may be characterized as laminated structures of carbon, that is, structures consisting of superposed layers or laminae of carbon atoms joined together by weak van der Waals forces. In considering the graphite structure, two axes or directions are usually noted, to wit, the “c” axis or direction and the “a” axes or directions. For simplicity, the “c” axis or direction may be considered as the direction perpendicular to the carbon layers. The “a” axes or directions may be considered as the directions parallel to the carbon layers or the directions perpendicular to the “c” direction. The graphites suitable for manufacturing flexible graphite sheets possess a very high degree of orientation.
As noted above, the bonding forces holding the parallel layers of carbon atoms together are only weak van der Waals forces. Natural graphites can be chemically treated so that the spacing between the superposed carbon layers or laminae can be appreciably opened up so as to provide a marked expansion in the direction perpendicular to the layers, that is, in the “c” direction, and thus form an expanded or intumesced graphite structure in which the laminar character of the carbon layers is substantially retained.
Graphite flake which has been chemically or thermally expanded and more particularly expanded so as to have a final thickness or “c” direction dimension which is as much as about 80 or more times the original “c” direction dimension, can be formed without the use of a binder into cohesive or integrated sheets of expanded graphite, e.g. webs, papers, strips, tapes, or the like (typically referred to as “flexible graphite”). The formation of graphite particles which have been expanded to have a final thickness or “c” dimension which is as much as about 80 times or more the original “c” direction dimension into integrated flexible sheets by compression, without the use of any binding material, is believed to be possible due to the mechanical interlocking, or cohesion, which is achieved between the voluminously expanded graphite particles.
In addition to flexibility, the sheet material, as noted above, has also been found to possess a high degree of anisotropy to thermal and electrical conductivity and fluid diffusion, somewhat less, but comparable to the natural graphite starting material due to orientation of the expanded graphite particles substantially parallel to the opposed faces of the sheet resulting from very high compression, e.g. roll processing. Sheet material thus produced has excellent flexibility, good strength and a very high degree or orientation. There is a need for processing that more fully takes advantage of these properties.
Briefly, the process of producing flexible, binderless anisotropic graphite sheet material, e.g. web, paper, strip, tape, foil, mat, or the like, comprises compressing or compacting under a predetermined load and in the absence of a binder, expanded graphite particles which have a “c” direction dimension which is as much as about 80 or more times that of the original particles so as to form a substantially flat, flexible, integrated graphite sheet. The expanded graphite particles that generally are worm-like or vermiform in appearance will, once compressed, maintain the compression set and alignment with the opposed major surfaces of the sheet. Properties of the sheets may be altered by coatings and/or the addition of binders or additives prior to the compression step. See U.S. Pat. No. 3,404,061 to Shane, et al. The density and thickness of the sheet material can be varied by controlling the degree of compression.
Lower densities are advantageous where surface detail requires embossing or molding, and lower densities aid in achieving good detail. However, higher in-plane strength and thermal conductivity are generally favored by more dense sheets. Typically, the density of the sheet material will be within the range of from about 0.04 g/cm3 to about 1.9 g/cm3.
Flexible graphite sheet material made as described above typically exhibits an appreciable degree of anisotropy due to the alignment of graphite particles parallel to the major opposed, parallel surfaces of the sheet, with the degree of anisotropy increasing upon roll pressing of the sheet material to increased density. In roll-pressed anisotropic sheet material, the thickness, i.e. the direction perpendicular to the opposed, parallel sheet surfaces comprises the “c” direction and the directions ranging along the length and width, i.e. along or parallel to the opposed, major surfaces comprises the “a” directions and the thermal properties of the sheet are very different, by orders of magnitude typically, for the “c” and “a” directions.